7.6.1 Physiological disorders
Physiological disorders are adverse quality changes that occur in fresh produce because of metabolic disturbances. These disturbances can be caused by internal factors such as mineral imbalances or may be due to non-optimal environmental factors such as inappropriate storage temperatures or atmosphere composition. The symptoms may be unique to a particular condition on a specific produce type; however, in many cases the symptoms are similar in a range of conditions with differing underlying causes. Mild symptoms are often confined to superficial tissues which may not be too significant if the produce is to be processed, but can strongly decrease marketability of the fresh product owing to visual disfigurement. Furthermore, physiological disorders can increase the susceptibility of the commodity to invasion by pathogens. The onset of disorders may be determined by pre-harvest conditions, the cultivar, maturity or stage of ripeness.
Poor nutrition will generally give rise to poor field growth and field symptoms. There are, however, a number of nutritional imbalances, which have no obvious pre-harvest significance but which give rise to symptoms during post-harvest storage. One of the most important nutrients in this respect is calcium which plays an important role in maintaining cell wall stability. A classic example is 'bitter pit' in apples in which hard, sunken brown pits develop both on the skin and internally. Affected tissues have a slightly bitter taste.
There is a wide range of disorders relatedtoexposureofproducetotempera-tures which are too high or too low. Hightemperaturescaused, forexample,by excessive exposure to the sun or inappropriatepost-harvestheattreatments,may cause skin damage and uneven fruit ripening.Onlyafewcommoditiesdestined for fresh consumption can survive mild freezing, for example parsnip and onions, however, the majority of fruits and vegetablesdestinedforfreshconsumption cannot tolerate any freezing at all. Ice crystals form inside the cells leading to membrane rupture, and the tissue collapses upon defrosting leading to unacceptable textural changes. These changes arelessobvioustotheconsumerinproduce with a relatively low water content and/or which will be cooked before consumption, for example, peas, sweet corn,parsnips,potatoes,carrots,broccoliand spinach.
Chilling injury is quite distinct from freezing injury and may occur at temperatures well above freezing point (Saltveit and Morris, 1990). Tropical and subtropical commodities are particularly susceptiblealthoughtheremaybeconsiderable differences in chilling sensitivity betweencultivarsandbetweenimmatureand mature or unripe and ripe produce. Symptoms include water soaking, surface pitting, internal discoloration, failure to ripen, accelerated senescence and increased susceptibility to decay. Symptomsmaynotbecomeobviousuntilthe produce temperature has been raised to non-chilling levels. At temperatures below 8-10°C and maximal at about 2°C, Irishpotatoesaresusceptible toreversiblelow temperature sweetening (Burton, 1989). Thereducingsugarsproducedcauseprob-lems to the processing sector (see section 7.3.4 above).
If produce is stored in an atmosphere with insufficient oxygen or excessive carbon dioxide, for example in poorly ventilated stores, respiratory disorders can develop. At higher temperatures, the produce respires more quickly so that an unsuitable atmosphere can develop more rapidly. Symptoms depend on the product in question, so for example, potatoes may develop a black centre whereas lettuces may have pale midribs. Some apple cultivars suffer external injury and others develop internal browning owing to excessive carbon dioxide in the tissues. Very low oxygen levels can lead to alcoholic fermentation with accompanying off-odours. Tolerance levels are variable, for example, some apple cul-tivars tolerate levels less than 1% O2, whereas sweet potatoes are highly sensitive and fermentation may set in if O2 levels fall below 8%. Anaerobic conditions will also encourage the growth of soft-rotting bacteria in potatoes.
A range of specific symptoms in stored fruits and vegetables have been attributed to exposure to ethylene (Kader, 1985). Some examples include russet spotting of lettuce (at concentrations >0.1 ppm) which is associated with increased activity of phenylalanine ammonium lyase (PAL) and phenolic content, formation of the toxin pisatin in peas, and production of phenolics in sweet potatoes and in carrots. In carrots, the phenolic isocoumarin gives a bitter flavour and bitter flavours have also been noted in beetroot.
There are also a number of well-defined miscellaneous disorders of certain fresh produce which are beyond the scope of this book. Further information can be found in books by Snowdon (Snowdon, 1990; 1991).
Physical injury is possibly the most important cause of loss in fresh produce. This is not due to the direct losses, although these can be significant in some crops but rather to the indirect effect of creating a wound in the surface of the produce. This wound is an ideal entry point for many post-harvest pathogens as described above. Injury also allows water loss which compromises the quality of the produce. Furthermore, physical injury stimulates ethylene production in plant tissues, which can lead to premature yellowing or ripening of commodities.
Physical injury can arise at any stage of the life of the crop, from insect injuries in the field to poor post-harvest handling. Many fungi invade through the stem end where the produce was severed from the mother plant. Poor packaging can create problems from cuts caused by sharp edges or hard parts of adjacent produce, for example pineapple crowns, to grazes caused by lack of padding or underfilling of cartons allowing movement of produce within the pack during transport and handling. Bruising can occur from dropping or compression bruising can occur if produce is stacked too high or packs are overfilled. Significant levels of wastage occur in the potato industry owing to internal bruising of potato tubers during storage and handling (Balls et al., 1982). The shelf-life of many fresh products is considerably reduced by physical damage caused by rough handling at the retail level, particularly where the produce is loose and can be 'picked over' by the potential customer.
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